Unless otherwise indicated herein, the materials described in this section are not prior art to the claims and are not admitted to be prior art by inclusion in this section.
In the aerospace industry, structural fasteners such as rivets are commonly used to join a structure such as metal sheet components. In an example, rivets are used for construction of primary structures of aircraft (e.g., fuselage, wings, and tail), as well as secondary structures (e.g., rudders). Rivets commonly are used for fastening an aerodynamic skin to a frame to provide a strong aerodynamically smooth surface. Further, rivets are also commonly used in the interior structure of aircrafts since rivets provide a light and secure method of fastening structural components together.
Before being installed, a rivet typically consists of a cylindrical shaft with a head on one end and a tail on the other end (commonly referred to as the buck-tail). The installation process for installing rivets to join a structure typically involves use of a rivet gun and a bucking bar. In particular, a typical rivet-installation process involves forming a hole in the structure and then placing the rivet in the rivet hole. The rivet gun is placed on one side of the rivet and the bucking bar is placed on the opposite side of the rivet. The rivet gun then hammers on the rivet and some of the force of the rivet gun is absorbed by the bucking bar. Under this force, each end of the rivet is compressed causing outward expansion of the rivet such that the rivet fills the rivet hole. Typically, the rivet is compressed until the rivet establishes a tight fit, which is commonly called an interference fit. Further, during installation, the tail is deformed, so that it expands (e.g., to about 1.5 times the original shaft diameter), thereby securely holding the rivet in place.
A rivet is typically sized for the thickness of the structure which it is to join and the stress which it is to carry. Further, the impact energy of the rivet gun is typically designed to completely form the button end on the tail of the rivet and cause the desired degree of interference between the rivet shank and the hole, and/or between the rivet head and the surface of the structure.
However, the typical rivet-installation process has a number of drawbacks. For instance, the typical rivet-installation process creates impact energy that propagates through not only the rivet but also the structure into which the rivet is being installed. In practice it is extremely difficult to precisely control the propagation of the impact energy throughout the system. The lack of control over the propagation of the impact energy throughout the system may lead to a rivet that fails to meet the desired degree of interference. In the typical rivet-installation process, when a rivet gun impacts a rivet, the impact energy creates an impact wave that travels through the rivet and hits the bucking bar. Much of this impact energy is transferred to the rivet thereby leading to the deformation of the rivet. However, the impact energy of the rivet gun is also transferred or dissipated in various other ways. For example, typically some of the impact energy is lost (e.g., as heat), some of the impact energy is transferred to the bucking bar, some of the impact energy is transferred to the rivet, and some of the energy is transferred to the structure being joined. Since it is difficult to precisely control the propagation of this impact energy, an undesired amount of energy may be transferred to the structure and/or the rivet. Thus, the traditional rivet-installation process often results in rivets that fail to precisely meet a desired degree of interference.
Another drawback of the traditional rivet-installation process is that the typical rivet installation process involves a large amount of human feedback. For instance, the typical rivet process involves a highly skilled operator to produce quality rivets consistently. Further, the typical rivet process involves highly skilled quality control inspectors to confirm that installation of rivets meet particular specifications of flushness, interference and button formation.
Yet another drawback of the traditional rivet-installation process is that the typical rivet installation process is unsuitable for joining structures such as composite materials. In the aerospace industry, the use of components including composite materials is widespread. However, currently it is extremely difficult to use rivets to join composite materials, due to the forces that the traditional rivet process imparts on the composite material. As mentioned above, the impact energy created by the rivet gun is often transferred to the structure to be joined. Since composite materials typically cannot sustain the forces of the standard rivet-installation process, rivets are not commonly used to join composite materials.